1. Technical Field
Techniques are disclosed enhancing well placement using ultra deep resistivity tools in logging while drilling (LWD), measurement-while-drilling (MWD), and directional drilling (Geo-steering) applications. The disclosed techniques include methods for locating the most conductive formation region relative to the tool, determining distances and relative directions of formation layer boundaries, whether a tool is approaching a layer boundary from above or below the layer boundary, whether a formation is planar or non-planar, and superposition techniques for evaluating complex non-planar formations in real time without carrying out multi-dimensional modeling calculations.
2. Description of the Related Art
An alternative to wireline logging techniques is the collection of data on downhole conditions during the drilling process. By collecting and processing such information during the drilling process, the driller can modify or correct key steps of the operation to optimize performance. Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation are known as measurement-while-drilling (“MWD”). Similar techniques focusing more on measurement of formation parameters than on movement of the drilling assembly are known as logging-while-drilling (“LWD”). However, the terms MWD and LWD are often used interchangeably, and the use of either term in this disclosure will be understood to include both the collection of formation and borehole information, as well as data on movement and placement of the drilling assembly.
Boreholes are frequently drilled horizontally in petroleum reservoirs to increase the drainage area, or the length of the borehole passing through the reservoir. Because petroleum reservoirs are typically located in layered earth formations, the position of a horizontal borehole with respect to the boundaries of the formation layer will often affect the productivity of the borehole. Specifically, water is heavier than hydrocarbons and therefore is disposed below hydrocarbons in a formation layer, or towards the bottom layer boundary. Hence, it is advantageous to drill or land the borehole near the top or upper layer boundary as opposed to the bottom layer boundary. Conversely, when placing a drain-hole or water disposal well, it is advantageous to place the well near the bottom layer boundary. The estimation of distances to layer boundaries, both top and bottom, is therefore important for production well landing and drain-hole positioning.
Various techniques for estimating the borehole position with respect to layer boundaries include those based on well logging measurements made for nearby or “offset” boreholes. These techniques assume that the composition and the geometry of the formation layers proximate to the borehole of interest are substantially the same as in the offset boreholes. Often, this assumption leads to inadequate results.
Other techniques are based on the observation of features, referred to as “horns,” which appear in measurements made by electromagnetic-type well logging instruments. When an electromagnetic instrument approaches a layer boundary with a large contrast in resistivity, a significant distortion of the resistivity signal magnitude, known as a horn, occurs. Qualitative estimates of the distance between the instrument and the layer boundary may be made by observing the magnitude of the horn.
Measurement-while-drilling (MWD) tools are available to guide drill strings and therefore the resulting boreholes into more productive reservoir zones. MWD tools used for this purpose typically have been propagation resistivity tools with a 360° measurement and deep imaging capability to detect fluid contacts and formation changes up to 15 feet from the borehole. However, the propagation resistivity MWD tools are non-azimuthal, and therefore do not indicate whether formation boundaries are above or below the tool. To compound this problem, most propagation resistivity measurements do not extend far enough beyond the tool to warn the driller in time to avoid drilling out of the intended reservoir layer. As a result of these deficiencies, the use of propagation resistivity MWD tools can lead to poor well placement.
As an improvement over propagation resistivity MWD tools, Schlumberger developed the PERISCOPE™ 15 deep imaging MWD tools, which incorporate tilted and transverse antennas in the drilling collar. The non-axial antennae obtain directional electromagnetic measurements that are used to determine distance and azimuthal orientation of formation boundaries in any type of mud. These measurements are transmitted uphole and displayed on a graphical interface to provide information on distance to boundaries, formation resistivity and orientation.
One of the important aspects of the PERISCOPE™ 15 technology is that the directional phase shift and attenuation sign change depends on whether the conductive shoulder layer is above or below the layer where the tool is located. This so-called polarity change can be used for geo-steering purposes. Further the PERISCOPE™ 15 tools can be used to determine bedding orientation to indicate how much the boundary is tilted to the right or to the left with respect to the up direction. However, this definition conveys no information as whether the tool string is approaching the reservoir boundary from above or below the boundary.
Therefore, a need exists for more accurate methods of well placement, which provides directional measurements while drilling so steering decisions can be made to place the borehole optimally in the reservoir of interest. In order to accomplish improved well placement, the distance to the boundaries of a reservoir and whether the drill string is approaching the reservoir boundary from above or below must be known. Because non-planar formation geometry is often encountered, two-dimensional and three-dimensional modeling is required, which is time-consuming and not suitable for real time interpretation or MWD. Accordingly, a method for evaluating non-planar formation geometry is needed that does not require time-consuming two-dimensional or three-dimensional modeling.